Cellulose is included in the cell walls of plants, exocrine secretions of microbes, mantles of sea squirts, and the like. Cellulose is a polysaccharide wherein the number thereof is the largest known on earth, and cellulose is a biodegradable material, has high crystallinity and is excellent in stability and safety, and cellulose has gained attention as an environmentally friendly material. Accordingly, cellulose is expected to be applied and developed in various fields.
Cellulose has strong intramolecular hydrogen bonding and high crystallinity. Therefore, cellulose hardly dissolves in water nor general solvents, and studies have been performed to increase the solubility thereof. Among them, a method has gained a great deal of attention in recent years wherein a primary hydroxyl group existing at the C6 position among three hydroxyl groups of cellulose is merely oxidized to form an aldehyde group or a ketone group and then the group is changed to a carboxyl group by the catalyst system of TEMPO (2,2,6,6-tetramethyl piperidin oxyradical), since the method can oxidize merely a primary hydroxyl group selectively and can be performed under mild conditions such as aqueous solvents and ordinary temperature. Furthermore, when TEMPO oxidation is performed on natural cellulose, it is possible to merely oxidize the crystal surface thereof in nano-scale order, while the crystallinity of cellulose is maintained. It is known that fine modified cellulose can be dispersed in water merely by cleaning, dispersing in water and performing simple mechanical treatment. For example, an invention is described in Patent Document 1 wherein cellulose is oxidized by a TEMPO oxidation reaction, and forms fine cellulose due to the subsequent mechanical treatment.